1. Field of the Invention
The present invention relates generally to wavelength-division-multiplexed fiber optic communication systems, and more specifically, to the elimination of polarization dependence in wavelength filters.
2. Description of Related Art
The enormous bandwidth of optical fibers currently cannot be fully utilized by employing a single data stream, because this bandwidth exceeds that of the transmitter and receiver electronics required to interface the fiber to external electronic systems. For this reason, various types of multiplexing are being considered for increasing the amount of information that can simultaneously be transmitted over a single fiber cable. Wavelength-division-multiplexing (WDM) is straightforward to implement and offers an extremely flexible way to configure optical fiber networks for telecommunications, data communications, and sensor systems. In the WDM approach, several data streams of moderate speed are carried on a single fiber by encoding each data stream with light of a different wavelength. In order to implement WDM, wavelength-tunable transmitters and/or receivers are required. In general, the receivers must incorporate tunable filters which satisfy the criteria that:
(1) the filter must have rapid tunablility (appreciably faster than one microsecond) for many applications, to allow for rapid network reconfiguration; PA1 (2) the filter must have narrow optical passbands (less than 15 nm), so that many data streams, each at a different wavelength, can be simultaneously transmitted within the wavelength range over which the fiber operates; PA1 (3) polarization-independent filtering is required, so that fiber-induced polarization fluctuations of the optical datastreams do not cause noise in the received signal; and PA1 (4) the filter should select only a single data stream so that the filter removes light only at a single wavelength, while permitting light at other wavelengths to pass unaltered through the device (known as a "channel dropping filter").
Wavelength filters for fiber optics are generally fabricated from either bulk optical components (gratings, interference filters, etc.) or optical fibers (eg: fiber Fabry-Perot resonators). These devices can be polarization independent, but can only be tuned slowly (because tuning is performed by mechanically adjusting the device). Slow mechanical tuning is unsuitable for many WDM networks. Fast tunability is achievable only by using electro-optic or acousto-optic effects, which in turn require waveguide device geometries in order to minimize the magnitude of the tuning signals. Devices without waveguides typically exhibit short interaction lengths, and thus require very high voltages, currents, or acoustic powers in order to vary their wavelengths. However, the waveguide geometry required for fast tunablility also introduces polarization sensitive behavior. That is, the optical filter response depends on the polarization of the input signals due to waveguide birefringence phenomena. Such polarization dependence renders these components useless for most fiber communications, since fibers currently deployed for telecommunication and data communication do not preserve polarization, and polarization-preserving fiber systems are significantly more expensive. When used with non-polarization-preserving (ie: conventional) fibers, CDF polarization sensitivity converts random fiber polarization noise (due to microphonics, etc.) into amplitude noise, garbling the transmitted signal and making the link unusable.
Previous work on such channel dropping filters has often employed asymmetric waveguide directional couplers, in which light is transferred between two spatially-separated waveguides by a wavelength-dependent interaction. The basic concept of an asymmetric waveguide coupler (Alferness & Schmidt, Appl. Phys. Lett. 33, 161,1978 and U.S. Pat. No. 4,146,297; also Morishita et al, U.S. Pat. No. 5,031,989) is to bring two waveguides of different refractive index distributions and core thicknesses into proximity, such that light can couple from one waveguide to another. Because of the structural differences between the waveguides, this coupling occurs only for a specified wavelength. This means the device functions as a wavelength channel dropping filter. If several optical signals at different wavelengths are launched into a single-waveguide, the signal at one wavelength is coupled to the second waveguide of the coupler, while all other wavelengths remain in the same waveguide. The advantage of the waveguide-type design (as compared to bulk optics), is that it enables the use of a long device length, which in turn enables one to tune the filter by electro-optic effects. The electro-optic effects are generally so weak that a long interaction length is required for wide wavelength tuning range; without the waveguide structure, the optical signal would tend to diffract out of the filter and could not be effectively affected by the electrooptic tuning. This basic design has been demonstrated by several different research groups (Alferness & Schmidt, Appl. Phys. Lett. 33, 161, 1978, Broberg et al., J. Lightwave Technol. 4, 196, 1986, Wu et al., Photon. Technol. Lett. 3, 519, 1991).
FIGS. 1A-C shows a prior art filter. FIG. 1A shows the refractive index spatial profile n(x,y). FIG. 1B shows a top view of the two waveguides of the device. FIG. 1C shows the wavelength filtering response for two orthogonal polarizations ("TE" and "TM"). The filtering interaction can be the directional coupling phenomenon itself, or coupling assisted by a superimposed diffraction grating which can be structural (built-in), or generated by acousto-optic effects.
The diffraction grating can improve device performance, ie: reduce the filter passband or increase filter tunability. This has been described by Sakata and Takeuchi (Photon. Technol. Lett. 3, 899, 1991), Chuang et al. (Appl. Phys. Lett. 63, 880, 1993) and Alferness et al. (Appl. Phys. Lett. 55, 2011, 1993, and U.S. Pat. Nos. 4,737,007, 5,253,314, and 4,904,045).
In all cases, efficient coupling of light between the two waveguides requires that they be phase matched, that is, that their wavelength-dependent modal refractive indices n.sub.l,2 (.lambda.) be related by n.sub.1 (.lambda.)=n.sub.2 (.lambda.).+-.p .lambda./L where .lambda. is the free-space wavelength, p is an integer, and L is the grating pitch for grating-coupled devices. Because non-identical guides are used in the coupler, n.sub.1 and n.sub.2 differ, so that the above equation is satisfied and channel dropping occurs for only a single wavelength .lambda.CD. In general, waveguide birefringence results in different values of the indices n.sub.1,2 for different polarizations of the optical input. Moreover, the polarization dependence is different for n.sub.1 and n.sub.2, which in turn causes .lambda.CD, and thus the wavelength filtering, to be polarization-dependent. This behavior is shown schematically for a conventional device in FIG. 1C.
Integration of these coupler filters with additional optoelectronic devices such as lasers or photodetectors has been suggested and/or demonstrated by several researchers. Integration of these coupler filters with photodetectors has been demonstrated by: Sakata et al (Electron. Lett. 28, 749 1992 and U.S. Pat. Nos. 5,233,187 and 5,140,149), and Koch et al. (Appl. Phys. Lett. 51, 1060, 1987). Integration of these coupler filters with lasers has been demonstrated by: Nojiri and Sakata (U.S. Pat. No. 5,220,573), and Alferness et al (Appl. Phys. Lett. 60, 3209, 1992). Optical amplifiers and detectors have been integrated with coupler filters by: Chuang et al. (Appl. Phys. Lett. 63, 880, 1993) and Kim et al. (Photonics Technol. Lett. 5, 1319, 1993). Multiple-quantum well modulators have been used to create refractive index differences (electro-optic effects) which tune a selected filter wavelength by: Alferness et al. (U.S. Pat. No. 4,904,045), Sakata and Takeuchi (Appl. Phys. Lett. 59, 3081, 1991).
Heismann et al. (Appl. Phys. Lett. 64, 2335, 1994) have demonstrated a polarization-insensitive channel dropping filter using a double-periodic grating-assisted-coupler design. While this approach achieves polarization-insensitive performance, it also involves several undesirable features: it is restricted to grating-assisted devices, the double periodicity introduces secondary filter responses at undesired wavelengths, and the design increases the overall size (length) of the coupler device.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for designing polarization-independent optical wavelength filters for channel dropping filter (CDF) applications.
It is also an object of the present invention to provide polarization-independent optical wavelength filters for channel dropping applications.
The present invention obtains polarization-independent performance from directional coupler CDF devices. By introducing a birefringent material into one of the two waveguides of a directional coupler CDF device, material birefringence compensates for the "electromagnetic" waveguide birefringence. A layered composite material, with layer dimensions much smaller than a wavelength, operates as an equivalent bulk material which is birefringent. Replacing one of the waveguide materials with a layered composite achieves a polarization-independent device. This approach is implemented using III-V semiconductor materials, because these materials permit the CDF to be integrated with other optoelectronic components. The layered material used to obtain birefringence compensation is a so-called "multiple quantum well." Our use of multiple quantum wells in these devices serves a totally different purpose than their use in prior art, i.e., to enhance the electrooptic tuning of the filter wavelength by Alferness et al. (U.S. Pat. No. 4,904,045) and Sakata and Takeuchi (Appl. Phys. Lett. 59, 3081, 1991). The prior art does not meet our birefringence cancellation conditions, and prior art devices containing multiple quantum wells remained highly polarization-sensitive, as described by Sakata and Takeuchi (Appl. Phys. Lett. 59, 3081, 1991).
This invention is not restricted to III-V semiconductor materials; however, and also applies to other materials in which layered composites can be formed. A second example involves the use of composites consisting of alternating layers of different glasses (eg: TiO.sub.2 and SiO.sub.2), which can be deposited by chemical vapor deposition or sputtering. This invention also applies to grating-assisted couplers, in which either an etched grating is inserted between the two interacting waveguides or an acousto-optically generated grating is used to phase-match the waveguides.
Because this invention permits polarization-independent filtering while maintaining a waveguide coupler structure for the filter, it will meet all the performance criteria outlined above, i.e., it will provide a narrow filter bandwidth, fast filter tuning and channel dropping, without compromising polarization independence.